The artist is seeing his work mounted in museums in Berlin and New York and at a gallery in London

When "Evidence," Chinese artist Ai Weiwei's biggest exhibition to date opened in Berlin last week, one person was notably missing from the guest list: the artist himself.

Barred from traveling abroad since his release from detention in 2011, Mr. Ai couldn't be in Germany—or in New York. There, his first museum show in the city opens at the Brooklyn Museum April 18 and runs through Aug. 10.